A valve or a gate is provided in a fluid passage through which a fluid such as a water flows, and is opened and closed to thereby regulate a flow of the fluid. In general, the valve or the gate is automatically opened and closed by an actuator in which an electric motor, a hydraulic motor or the like is used as a drive power source. When the valve or the gate is large, in order to obtain a necessary drive force, an output of the electric motor is reduced speed and utilized.
In many cases, a worm gear mechanism is utilized as a reduction mechanism in the actuator for the valve or the gate because it is possible to easily obtain a large reduction ratio, needed to smoothly drive the valve or the gate, by the worm gear mechanism at the minimum necessary reduction steps, and because the worm gear mechanism features a self-lock property for preventing the input side of the worm gear mechanism from being moved when a force is exerted on the output side thereof, so that an accidental movement of the valve or the gate is prevented when a driving force resulted from a flowing fluid, the force of gravity or the like is exerted on the valve or the gate.
Nevertheless, since the worm gear mechanism in which the worm and the worm wheel are in slidable contact with each other for transmission of a rotational force has a lead angle giving the self-lock property, the worm gear mechanism has an efficiency of force transmission which is inferior to that of spur gears so that there is a demand of improvement of the force transmission efficiency in the worm gear mechanism. Conventionally, in order to improve the force transmission efficiency in the worm wheel mechanism, although the gear faces have been subjected to specific processes, treatments or the like, contrived lubrication and so forth, these ides could not contribute to a great improvement of the force transmission efficiency.
Also, it has been proposed that a ultrasonic vibration is exerted on either the worm or the worm wheel so that friction on the gear faces can be decreased, in resulting in improvement of the force transmission efficiency in the worm gear mechanism. An example of this prior art worm gear mechanism is disclosed in JP-08-145126 A.
In the aforesaid prior art worm gear mechanism, the ultrasonic vibration generated by a ultrasonic oscillator supersonic is amplified and transmitted to the worm wheel engaged with the worm. When the worm is rotationally driven to thereby rotate the worm wheel, a coefficient of friction between the gear faces is decreased in comparison with the case where the ultrasonic vibration is not exerted on the worm wheel, resulting in an considerable improvement of the force transmission efficiency in the worm gear mechanism.